Beam splitter comprising colour separating planoparallel plates



SEARCH ROOM June 9, 1959 A. WIEBERDINK BEAM SPLITTER COMPRISING COLOURSEPARATING PLANOPARALLEL PLATES Filed Jan. 21, 1955 INVENTOR ATEwqssanomx fl? AGENT United States Patent BEAM SPLITTER COMPRISING COLOURSEP- ARATIN G PLAN OPARALLEL PLATES Ate Wieberdink, Emmasingel,Eindhoven, Netherlands, asslgnor, by mesne assignments, to NorthAmerican Philips Company, Inc., New York, N.Y., a corporation ofDelaware Application January 21, 1955, Serial No. 483,384

Claims priority, application Netherlands February 6, 1954 3 Claims. (Cl.88-1) Beam splitters comprising colour separating planoparallel platesto break the light emanating from a source of light into a plurality ofcolour components are used, among other things, in colour television. Inthis case the light emanating from an object is broken up into threefundamental colours. With the aid of a camera the light of a particularfundamental colour is converted into an electric signal; the threesignals thus obtained or the linear combinations thereof can beconverted again in a suitable receiver into a picture of the object inits natural colours.

Such beam splitters are, in general, composed of two intersecting,colour separating planoparallel plates, each of which transmitssubstantially completely particular parts of the visible spectrum andeach of which reflects substantially completely other parts thereof.

These beam splitters have a limitation in that at the intersection ofthe two planoparallel plates the effect of the colour separation isdisturbed, which, in the finally reproduced picture, produces atroublesome ribbon, whose breadth is directly proportional to the ratiobetween the thickness of the planoparallel plates and their length,measured in a direction at right angles to their line of intersection.In order to obviate this unwanted phenomenon, it has been suggested tocompose the beam splitter of four prisms joined together in a mannersuch that the separation planes constitute two intersecting surfaces,each of which is provided with a material producing the colourseparation.

In the situation involving separation of the light emanat ing from anobject into three colour components, two aspects may be distinguished,i.e.

(1) It may be required that the pictures produced by the beam splitterinthe three fundamental colours should not be deformed,

(2) It is not required that the pictures should be undeformed at leastas far as their geometrical association is concerned.

The first-mentioned case may for example apply to the production of apicture in a particular fundamental colour on the photo-electric cathodeof a television camera operating by electron beam scanning. It will beobvious that, if such a picture is deformed, also the image finallyreproduced by a receiver will be deformed.

The second case applies for example to the scanning of a colour film bymeans of a white light beam, the passing light being separated intothree fundamental colours. The light of a particular fundamental colouris captured by a photo-electric cell. In this case, however, thegeometrical coherence of the pictures produced on the photo-electriclayer of the cell is not essential.

In both cases the beam splitter comprising two intersecting, colourseparating planoparallel plates exhibit the disadvantage describedabove; in both cases the beam splitter comprising four prisms obviatesit.

It will, however, be obvious that this solution is comparatively costly,since the four prisms must be ground accurately to fit exactly to oneanother; they must, more- Hce over, be satisfactorily achromatic inorder to avoid colour dispersion in the prisms themselves.

The invention has for its object to provide a materially simplersolution to avoid the occurrence of a dark ribbon in the image to befinally reproduced in the second case, i.e. if the images in thefundamental colours, as far as their geometrical coherence is concerned,are not required to be undeformed, and has the feature that the beamsplitter is composed of two parts separated from one another in space,each part comprising a V-shaped body of transparent material, which maybe subdivided, the limbs of each V-shaped body being constituted byplanoparallel plates, the peaks or apices of the bodies facing oneanother and the plane in which lie both the line of intersection of theouter surfaces of one V-shaped body and the line of intersection of theinner surfaces of this body and the corresponding plane of the otherV-shaped body lie at least substantially in the same plane, the colourseparating material, at least with the part located on the front side ofthe beam splitter, being applied to those sides of the planoparallelplates which face the front side of the beam splitter.

The invention will be described more fully with reference to theaccompanying drawing, in which Figs. 1 and 2 show embodiments of knownbeam splitters,

Fig. 3 shows one embodiment of a beam splitter according to theinvention, and

Fig. 4 serves to explain the invention.

Fig. 1 shows a sectional view of a known beam splitter, composed of twointersecting planoparallel plates 1 and 2, having layers of colourseparating material 3 and 4. Such a layer may for example be composed ofone or more layers of dielectric material, the thickness of the layerbeing required to be in a given proportion to the wavelengths of thepart to be transmitted and the part to be reflected of the lightspectrum. For example the layer 3 can reflect red light, and pass blueand green light; the layer 4 can reflect blue light and pass red andgreen light. The plates 1 and 2 serve as supports for the colourseparating material; they must of course be transparent and are mostlymade of glass.

The fact that the crossing of the two supports, one of which comprisestwo parts (the support 2 of the figure) has a disturbing efiect, isillustrated by two light beams 5 and 6. The light beam 5 is broken up bythe layer 3 into a reflected part 5,, containing mainly red light, and apassed part 5 containing mainly green and blue. The first part passesunhindered the layer 4 and the layer 2, apart from the refraction in theplaneparallel plate 2, and the second part, subsequent to its passagethrough the plate 1, strikes the layer 4. This layer breaks the lightfrom the plate 1 into a reflected part 5 containing mainly blue light,and a passed part 5 containing mainly green light. Upon considering thelight beam 6, it is evident from the figure that the passed part of thelight striking the layer 3 is no longer broken up into a green part anda blue part and that this part leaves the beam splitter, moreover, in anunwanted direction.

These phenomena do not occur with another known beam splitter shown inFig. 2. This beam splitter comprises four prisms 1, 2, 3 and 4, thecolour separating material being applied to the surfaces at which twoprisms engage one another. The layer 5 may, for example reflect redlight and pass blue and green light;

avoid unwanted colour dispersion in the prisms themselves. However, thissolution is quite costly.

Fig. 3 shows one embodiment of a beam splitter according to theinvention. It comprises two elongated V-shaped parts 1 and 2, separatedfrom one another in space and each constituted by two planoparallelplates of transparent material, i.e. part 1 by plates 3 and 4 and part 2by plates and 6. The plates 3 and 4 intersect one another in a plane 7of intersection, and the plates 5 and 6 in a plane of intersection 8.These intersection planes 7 and 8 are each also defined by the two linesformed by the inner intersecting and the outer intersecting surfaces ofthe V-shaped bodies 1 and 2. These planes of intersection need notcorrespond to the material boundaries of the planoparallel plates 3 and5 or 5 and 6 respectively, although this is to be preferred, since allplates 3, 4, 5 and 6 may have equal dimensions and the said surfaces maybe ground at the same angles; moreover, the application of the colourseparating layers is then most simple. However, one part may be made ofone piece or of two pieces as indicated in Fig. 3 for part 2. The twoplanes of intersection 7 and 8 form at least substantially a commonplane; the peaks or apices 9 and 10 of the V-shaped parts 1 and 2 faceone another; d designates the distance between the corners 9 and 10 ofthe parts 1 and 2. This distance may be zero without any objection. Thecolour separating or color selective material is applied in layers 11,12, 13, and 14 to those sides of the planoparallel plates which face thefront side of the beam splitter. The incident light on the front side isdesignated by the horizontal arrow to the left of the beam splitter.

This is required for the part 2 lying on the front side i.e. the partlying on the side where the light from the light source comes in, if atleast the occurrence of components emanating at an unwanted angle shouldbe avoided. This is not required for the other part 1, but theapplication of the colour separating layers 12 and 13 to the sides ofthe planoparallel plates 3 and 4 facing the front side has the advantagethat the light reflected by the layers 12 and 13 need not traverse twicethe plates 3 and 4. The layers 11 and 12 may for example reflect redlight and allow green and blue light to pass, and the layers 13 and 14reflect blue light and allow green and red light to pass.

The colour splitter according to the invention does not exhibit thedisadvantages inherent in the colour separator shown in Fig. 1, and itis of materially simpler construction than the beam splitter shown inFig. 2. The angles at which the planoparallel plates of the separateparts intersect one another, i.e. the angles a and a which arepreferably 90 each, need not be equal to one another whereas it will beobvious that the tolerances for the angles b of the prism (see Fig. 2)are within very narrow limits.

The fact that the beam splitter according to the invention does notexhibit the disadvantages inherent in the beam splitter shown in Fig. 1is illustrated in Fig. 4. Each of the incident light rays 1, 2 and 3 isbroken up, independent of the impact area on the beam splitter, into agreen component 1 2 and 3, respectively, a blue component 1 2 and 3;,respectively and a red component 1,, 2,. and 3 respectively, while nocomponents emerges at an unwanted angle, with the result that nolight-sensitive element is struck as in the case with the light ray 6 ofFig. 1.

From Fig. 4 it is evident that the emerging red and blue light beams donot contain rays in the hatched parts. This means that the geometricalcoherence of the images produced by these beams of the object isinterrupted. In many cases this is not an objection. Reference has beenmade to the case in which a colour film is scanned by means of a whitebeam, the components produced by the splitting action of the beamsplitter being captured each by a photocell. A further example is givenby the case in which images are produced by a beam splitter on thephoto-electric cathode of a television camera tube for light scanning,the light spots required for scanning being images of the same lightsource describing a scanning frame under the action of suitabledeflection means, these images being also formed via the same beamsplitter. Since the geometrical coherence both of the images of theobject and of the images of the light spot describing the scanning frameis interrupted, however in completely the same manner for the two, sothat the local relationship between the scanning frame and the picturesremains the same, this does not affect adversely the picture to befinally reproduced. As used herein' with reference to the plates of thebeam splitter, the term planoparallel connotes that theoppositely-facing surfaces of the plates are planar and parallel to oneanother.

What is claimed is:

1. An optical device adapted for separating incident light into aplurality of emergent light beams containing different color componentsor vice versa, comprising a pair of elongated, substantially V-shaped,in cross-section, bodies each comprising a pair of substantiallytransparent, planoparallel plates joined at their edges to form the apexof the V, said pair of bodies being mounted adjacent one another withtheir apices aligned with and facing one another and with their platesfacing outwardly in opposite directions to form, as a whole, a generallyX-shaped configuration, the lines defined by the inner intersecting andthe outer intersecting surfaces of the planoparallel plates of each ofsaid bodies each defining a plane, the thus defined planes of bothbodies being substantially coplanar, first and second layers of lightreflective and transmissive, color-selective material on the two innersurfaces of one of said bodies, and third and fourth layers of lightreflective and transmissive, color-selective material on two V-shapedsurfaces of the other of said bodies, said first and second layerspossessing different reflective and transmissive properties to differentcolor components but each of them possessing the same reflective andtransmissive properties as that of the third and fourth layers extendingin the same general direction and thus all the layers cooperating toseparate incident light into the differently-colored light beams.

2. An optical device as set forth in claim 1 wherein one of the V-shapedbodies is constituted of a pair of members joined along the said planedefined by the lines formed by the inner intersecting and the outerintersecting surfaces.

3. An optical device adapted for separating incident light into threeemergent light beams containing different color components or viceversa, comprising a pair of elongated, substantially V-shaped, incross-section, bodies each comprising a pair of substantiallytransparent, planoparallel plates joined at their edges to form the apexof the V, the angles included between the plates of each of the bodiesbeing about the same, said pair of bodies being mounted adjacent oneanother with their apices aligned with and facing one another and withtheir plates facing outwardly in opposite directions to form, as awhole, a generally X-shaped configuration, the lines defined by theinner intersecting and the outer intersecting surfaces of theplanoparallel plates of each of said bodies each defining a plane, thethus defined planes of both bodies being substantially coplanar, firstand second layers of light reflective and transmissive, color-selectivematerial on the two inner surfaces of one of said bodies, and third andfourth layers of light reflective and transmissive, color-selectivematerial on the two outer V-shaped surfaces of the other of said bodies,said first and second layers possessing different reflective andtransmissive properties to different color components but each of thempossessing the same reflective and transmissive properties as that ofthe third and fourth layers extending in the same general direction andthus all the layers cooperating to separate incident light into thediflerenfly-colored light beams.

References Cited in the file of this patent 6 Schroeder Iune16, 1953Rehorn Feb. 23, 1954 Sachtleben et a1. Mar. 16, 1954 FOREIGN PATENTSFrance June 20, 1905 France Mai. 20, 1926

